A known step apparatus for a vehicle is disclosed, for example, in JP2011-183834A which will be hereinafter referred to as Reference 1. The step apparatus disclosed in Reference 1 is made of carbon-fiber reinforced plastic. The aforementioned step apparatus includes an outer frame portion which at least partially extends along an outline of a planar form of the step apparatus and a beam portion which is connected to an inner side of the outer frame portion to extend from a vehicle inner side to a vehicle outer side. Each of the outer frame portion and the beam portion includes a substantially C-shape cross section. The step apparatus further includes a guide rail which is integrally formed for guiding a slide door in an opening and closing direction thereof. Accordingly, the step apparatus may be reduced in weight by 50% as compared to a case where the apparatus is made of steel while ensuring rigidity and strength substantially equal to steel.
A further reduction in weight is still desired for the step apparatus disclosed in Reference 1. At this time, it is considered to reduce a thickness of the step apparatus entirely or partially and to form a reinforcing rib at a rear side of a portion where strength is required. Nevertheless, an optimization design of such configuration may have limitations. In addition, in the step apparatus, plural attachment bosses are integrally formed at a portion closer to the vehicle inner side so that the step apparatus is mounted to a vehicle body via the attachment bosses. As a result, the step apparatus is fixed to the vehicle body in a cantilever beam manner. Thus, in a case where a user places one's foot to weight on a portion of the step apparatus closer to the vehicle outer side when the user gets in or out of a vehicle, a large load may be applied locally to an end of the portion fixed to the vehicle body in the cantilever beam manner. In order to overcome the foregoing drawback, a large reinforcement may be desired at the aforementioned portion, however, the large reinforcement is a trade-off from the weight reduction.
A need thus exists for a step apparatus for a vehicle which is not susceptible to the drawback mentioned above.